1. Field of the Invention
The present invention relates to an improvement in a direct-current brushless motor, and more particularly, to an improvement in the motor of the type wherein a current is applied to windings provided around a stator through an electrification control unit to drive a rotor.
2. Description of the Prior Art
FIG. 1(a) is an illustration of a conventional DC brushless motor, wherein numeral 1 represents a rotor shaft, 2 a rotor hub fixed to this rotor shaft 1, 3 a cylindrical permanent magnet which is fixed to the rotor hub 2 and which has N- and S-poles magnetized alternately around the circumference thereof, 4 a gap, 6 a stator yoke opposite to said rotor hub 2 across the gap 4, 5 a pole shoe provided at each of magnetic poles provided radially on the stator yoke 6, 7 a stator winding, 9 a magnetic detector detecting the magnetic flux of the permanent magnet 3, 10 a bearing of the rotor shaft 1, 11 a bracket holding the bearing 10, and numeral 8 a base plate supporting the bracket.
The rotor hub 2, which is made of a magnetic material, serves also as the yoke of the permanent magnet 3 and is supported rotatably by the bearing 10.
The relationship between the stator and the rotor is shown in a developed view in FIG. 1(b).
FIG. 1(b) illustrates an example wherein the permanent magnet 3 and the stator yoke 6 are each provided with four magnetic poles. That is, the stator yoke 6 is provided with magnetic poles 6-1 to 6-4 at equal pitches, and pole shoes 5-1 to 5-4 are formed at the ends of the magnetic poles 6-1 to 6-4, respectively, projecting to both sides of the magnetic poles 6-1 to 6-4, while the gap 4 between the shoes and the permanent magnet 3 facing them is fixed in size. Stator windings 7-1a to 7-4a and 7-1b to 7-4b, separated into two groups, are wound around the magnetic poles 6-1 to 6-4, respectively, each winding is separated into groups (a) and (b) and connected to form two sets of windings, which are connected to a driving circuit as shown in FIG. 1(d).
The rotational power of this motor is, as shown in FIG. 1(c), a resultant torque. To made up of a reluctance torque Tr produced by the changes in the magnetic resistance between the permanent magnet 3 and the pole shoe 5, and an electromagnetic torque Tm produced by the current flowing through the stator winding 7. The change is the magnetic resistance between the permanent magnet 3 and the pole shoe 5 is caused by variations in the area of the permanent magnet 3 that faces the pole shoe 5 according to the position of the magnet, even when the size of the gap 4 between them is fixed.
The areas of the N- and S-poles facing each pole shoe becomes equal, and the reluctance torque becomes zero, when the border line 3-1 between two N- and S-poles of the permanent magnet 3 is in alignment with the center line of the pole shoe 5-1 as shown in FIG. 1(b). This corresponds to point P in FIG. 1(c).
The reluctance torque Tr changes as shown in FIG. 1(c): it becomes zero at point P, is positive to the left of that point and negative to the right thereof, becomes a maximum value at a certain position, and becomes zero when between the pole shoes 5-1 and 5-2. Meanwhile, in the driving circuit shown in FIG. 1(d), the detection of an N-pole by the magnetic detector 9 makes a transistor Q.sub.1 conductive and this makes a current flow in the winding 7-a, generating N-poles in the pole shoes 5-1 and 5-3. At the same time, the parts of the permanent magnet having N-poles facing the N-poles thus generated repel them while the parts with S-poles attract them, and thus the permanent magnet is moved toward the right. S-poles are generated in the pole shoes 5-2 and 5-4 in the same way, the parts of the permanent magnet having S-poles which face the S-poles thus generated repel them, while the parts with N-poles attract them, and the permanent magnet is moved toward the right. When the border line 3-1 arrives at a position of the magnetic detector 9, the output of the magnetic detector 9 becomes zero, the transistor Q.sub.1 is turned off, and the electromagnetic torque Tm becomes zero at point S in FIG. 1(c). A transistor Q.sub.2 is then turned on to pass a current through the winding 7-b and generate the electromagnetic torque Tm. Accordingly, the resultant torque To of the motor shown in FIG. 1(a) has the waveform shown by the broken line in FIG. 1(c), and in the vicinity of point S which is the midposition of the pole shoes, there is a place where the resultant torque becomes zero, which brings about the defect that self-starting of the motor could be impossible, depending on the position of the rotor.